1. Field of the Invention
This invention relates to a developer for developing an electrostatic image, an image forming method, an image forming apparatus and an apparatus unit that are used to convert an electrostatic latent image into a visible image in image producing processes such as electrophotography, electrostatic recording and electrostatic printing.
More particularly, the present invention relates to an image forming method having the contact sharging step of carrying out electrostatic charging by bringing a charging member to which a voltage has been externally applied, into contact with an electrostatic image bearing member, the developing step of developing an electrostatic latent image formed on the electrostatic latent image bearing member by the use of a developer and the contact transfer step of transfer a developed image to a transfer medium while a transfer member to which a voltage has been externally applied is pressed against the electrostatic image bearing member. It also relates to a developer for developing an electrostatic image, used in such an image forming method, and an image forming apparatus and an apparatus unit that have such a developer for developing an electrostatic image.
2. Related Background Art
A number of methods as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publication Nos. 42-23910 and 43-24748 and so forth are hitherto known for electrophotography. In general, copies are obtained by forming an electrostatic latent image on a photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the latent image by the use of a toner, and transferring the toner image to a recording medium such as paper if necessary, followed by fixing by the action of heat, pressure, heat-and-pressure, or solvent vapor. The toner that has not transferred to and has remained on the photosensitive member is cleaned by various means, and then the above process is repeated.
In recent years, such copying apparatus are not only used as copying machines for office work to merely take copies of originals, but also has began to be used as information output machinery connected with other information processing machines as a result of introduction of digital techniques, as copying machines for preparing new originals as a result of the achievement of multi-function that has made it easy to process or edit image information, and also as personal copying machines for private use.
Hence, the apparatus are severely sought to be made more high-speed, to achieve a higher image: quality and to be made more small-sized and lightweight, and also severely sought to be more: highly reliable.
Under such circumstances, in printers or copying machines making use of electrophotographic techniques, corona dischargers have been commonly put into wide use i) as a means for uniformly charging the surface of a photoconductor (an electrostatic image bearing member) and ii) as a means for transferring a developer-developed image on the surface of the photoconductor. However, research and development have been made on a method in which direct charging and transfer are carried out by externally applying voltages while directly bringing charging members into contact with, or pressing them against, the surface of the photoconductor, and such a method is being put into practical use.
Such a method is disclosed, for example, in Japanese Patent Applications Laid-open No. 63-149669 and No. 2-23385. These are concerned with contact charging or contact transfer, where a conductive elastic roller is brought into contact with an electrostatic image bearing member, the electrostatic image bearing member is uniformly charged while applying a voltage to the conductive roller, which is then exposed to light to form thereon an electrostatic latent image, and thereafter, while pressing against the electrostatic image bearing member another conductive elastic roller to which a voltage has been applied, a transfer medium is passed between them to transfer to the transfer medium the toner image formed on the electrostatic image bearing member, followed by fixing to obtain a copied image.
An example of the image forming method having such contact charging and contact transfer systems will be described below with reference to a schematic illustration in FIG. 4.
Reference numeral 101 denotes a rotating drum type electrostatic image bearing member (hereinafter "photosensitive member"). The photosensitive member 101 is formed of layers basically comprised of a conductive substrate layer 101b made of aluminum or the like and a photoconductive layer 101a formed on its periphery, and is clockwise rotated as viewed in the drawing, at a given peripheral speed (process speed).
Reference numeral 102 denotes a charging roller, which is basically comprised of a mandrel at the center and a conductive elastic layer formed on its periphery. The charging roller 102 is brought into pressure contact with the surface of the photosensitive member 101 at a given pressure, and is rotates following the rotation of the photosensitive member 101. Reference numeral 103 denotes a charging bias power source through which a voltage is applied to the charging roller 102. Application of a bias to the charging roller 102 causes the surface of the photosensitive member 101 to be charged to a given polarity and potential. Image exposure 104 subsequently carried out gives formation of electrostatic latent images, which are successively converted into visible images as toner images through a developing means 105.
Reference numeral 106 denotes a transfer roller, which is basically comprised of a mandrel 106b at the center and a conductive elastic layer 106a formed on its periphery. The transfer roller 106 is brought into pressure contact with the surface of the photosensitive member 101 at a given pressure, and is rotated at a speed equal to, or different from, the peripheral speed of the photosensitive member 101. A transfer medium 108 is transported between the photosensitive member 101 and the transfer roller 106 and at the same time a bias with a polarity reverse to that of the toner is applied from a transfer bias power source 107, so that the toner image on the photosensitive member 101 is transferred to the surface of the transfer medium 108.
Subsequently, the transfer medium 108 is transported to a fixing assembly 111 basically comprised of a fixing roller 111a internally provided with a halogen heater and an elastic-material pressure roller 111b brought into pressure contact with it at a given pressure, and is passed between the rollers 111a and 111b, so that the toner image is fixed to the transfer medium 108 and the fixed image is outputted as an image-formed article.
From the surface of the photosensitive member 101 after the toner image has been transferred, contaminants such as untransferred toner remaining adhered thereto are removed to make the surface clean by means of a cleaning assembly 109 provided with an elastic cleaning blade counter-clockwise brought into pressure contact with the photosensitive member 101. The surface is then subjected to charge elimination through a charge elimination exposure assembly 110, and is repeatedly used for image formation.
Image forming apparatus having such contact charging and contact transfer systems enable uniform charging of a photosensitive member and satisfactory transfer therefrom at a bias with a relatively low voltage compared with corona charging and corona transfer, and is advantageous for making chargers themselves small-sized and preventing corona discharge products such as ozone.
When, however, a usual developer comprising toner particles comprised of a binder resin and a colorant such as a magnetic material, to which a fluidity-imparting agent such as silica is added, is used in the image forming apparatus having contact charging and contact transfer systems, the toner particles slightly remaining on the photosensitive member that have been unremoved in the cleaning step after transfer go through the charging roller and transfer roller brought into pressure contact with the photosensitive member and may stick fast to the surfaces of both the rollers and photosensitive member. With repetition of copying, such toner particles melt-adhere to be more strongly fixed, so that faulty charging, faulty cleaning and faulty transfer may be caused. Thus, there is the problem that a decrease and uneveness in image density, white dots in solid black images and black spots in solid white images are liable to occur in the images obtained.
Various methods such as the blade method, the fur brush method and the magnetic brush method are known as methods by which the toner remaining on the photosensitive member after transfer is removed. Under the existing conditions, however, it is impossible to completely remove by these methods the toner remaining on the photosensitive member after transfer.
For the purpose of preventing this phenomenon in which toners adhere to photosensitive members, Japanese Patent Application Laid-open No. 48-47345 suggests adding both friction-decreasing substance and an abrasive substance to a toner. Since, however, the friction-decreasing substance is a substance that forms an adherent filmy deposit, a film ascribable to the friction-decreasing material may be formed on the built-in charging roller or transfer roller if such a toner is used in the image forming apparatus having contact charging and contact transfer systems, bringing about the problem that faulty charging and faulty transfer may remarkably occur.
Organic photosensitive members (organic photoconductors) are also prevalent as photosensitive members used in medium-speed machines to make copying machines small-sized and to reduce cost. In particular, for the purpose of decreasing the wear of the surface layer of an organic photosensitive member to prevent deterioration of its charge performance, Japanese Patent Application Laid-open No. 63-30850 suggests an organic photosensitive member having a surface layer containing a lubricant such as a fine fluorine resin powder. The organic photosensitive member containing such a lubricant can certainly enjoy a longer lifetime of the photosensitive member itself.
However, because of a poor dispersibility of the lubricant in a binder resin such as polycarbonate that constitutes the surface layer, its use may conversely cause a decrease in smoothness of the surface of the photosensitive member, so that, if the photosensitive member is used in the image forming apparatus having contact charging and contact transfer systems, the toner after development may get into concaved portions on the surface to cause a great lowering of the cleaning performance when the remaining toner is removed in the cleaning step after transfer, tending to worsen the phenomenon of melt-adhesion of toner to the charging roller and transfer roller and to the photosensitive member.
In relation to the step of fixing toner images to transfer mediums such as transfer paper, the method most prevalent at present is the heat-pressure system using a heating roller, i.e., what is called the heat-roll fixing system. In the heat-roll fixing system, a waiting time is required until the heating roller reaches a given temperature. As this waiting time is made shorter and the apparatus is made higher in speed, faulty fixing tends to be caused by a drop of temperature of the heating roller due to the passage of the transfer medium, and instantaneous fixing on the transfer medium is liable to occur. Moreover, since the heating roller surface comes into contact with toner images in the state the toner is molten, a part of the toner images may adhere to the surface of a fixing roller, which is transferred thereto and may be further transferred to the subsequent transfer medium, so that the so-called offset phenomenon is liable to occur.
For this reason, improvements in fixing performance of the developer have been made, and a proposal is made in Japanese Patent Publication No. 63-32182 for the purpose of improving a low-temperature fixing performance of toners. In this publication, a toner containing as a binder resin component of the toner a vinyl polymer having at least one peak in each of specific regions of low-molecular weight and high-molecular weight is proposed, which contains the low-molecular weight component in a relatively large amount so that the low-temperature fixing performance can be improved. In another disclosure in Japanese Patent Application Laid-open No. 2-235069, a magnetic toner comprises a binder resin having two peak values in its molecular weight distribution, in which, setting a bottom value between both the peak values as a reference point, each of the dispersion ratios in the low-molecular weight region to that in the high-molecular weight region is made small to improve low-temperature fixing performance and anti-offset properties.
Although the toner as described above is improved in fixing performance, there is the problem that the use of such a toner in the image forming apparatus having contact charging and contact transfer systems tends to worsen the phenomenon of melt-adhesion of toner on the charging roller and transfer roller and on the photosensitive member.
Moreover, even if a cleaning member such as a felt pad or an elastic blade is set in contact with both the rollers so that contaminants such as paper dust produced from transfer mediums such as paper can be prevented from adhering to the charging roller and the transfer roller, the toner may so strongly melt-adhere to the both rollers that it is difficult to decrease faulty cleaning.